There is known a slide fastener which includes a pair of fastener tapes, a pair of core strings respectively provided on opposing tape side edge portions of the pair of fastener tapes, a pair of fastener element rows respectively provided on the pair of core strings, each of the pair of fastener element rows having a plurality of fastener elements made of metal, and a slider configured to engage and disengage the pair of fastener element rows.
It is important for the slide fastener to maintain the engaged state, and the slide fastener is required to exhibit a preset level of engagement strength in several tests. Examples of such tests include a test of measuring the strength of engagement between fastener elements by grasping a pair of fastener tapes which are engaged with each other and pulling the pair of fastener tapes in the direction in which the fastener tapes are away from each other (horizontal pulling strength test) or a test of measuring the strength of engagement by applying a force to the portions where the fastener elements are engaged in a direction from the backside to the front side with using a rod-shaped member (thrust-up strength test).
In order to improve the engagement strength, it is considered to increase the area where the fastener elements are engaged and in contact with each other by changing the shape of engagement sections of the fastener elements. However, increase in the contact area leads to increase in the size of the engagement sections, and thus there is a problem in that engagement or disengagement is not smoothly performed when the slider is operated.
On the other hand, the horizontal pulling strength or the thrust-up strength of the fastener elements can be improved by decreasing the distances between the fastener elements attached to the core strings of the fastener tapes to be smaller than typical distances. However, this case also has a problem in that engagement or disengagement is not smoothly performed when the slider is operated.
As such, improvement in the engagement strength is closely related to the operation of the slider, and their balance is important.
In addition, although distances (pitches) at which the fastener elements are attached are set considering the elasticity of the fastener tapes, when a force is applied to the fastener elements which are attached to the core strings of the fastener tapes, the distances increase in response to inclination or minute movement of the fastener elements, thereby decreasing the engagement strength. This is known through the studies of the inventor of this application.
There is known a slide fastener of the related art in which lacquer is applied to fastener tapes and core strings to stiffen the fastener tapes and the core strings, thereby preventing fastener elements from moving (e.g. see Patent Document 1).